Preparation of methyl acrylate



Patented Mar. 18, 1947 PREPARATION OF METHYL ACRYLATE Hugh J. Hagemeyer,Jr., Kingsport, Tenn., as-

signor to Eastman Kod N. Y., a corporation of ak Company, Rochester, NewJersey No Drawing. Application December 23, 1943, Serial No. 515,422

2 Claims. 1

This invention relates to improvements in the production of unsaturatedesters and nitriles by the pyrolysis of the esters, amides, or ammoniumsalts of the hydroxy monocariboxylic aliphatic acids in the presence ofketene.

It is known that unsaturated acids may be.

produced by the dehydration of monocarboxylic hydroxy acids and theiresters, or by the elimination of hydrogen halides from halogenderivatives of the monocarboxyli: acids. Difiiculties, however, havebeen experienced in previous processes of this nature. The preparationof a-methyl acrylic acid by the dehydration of a-hydroxyisobutyric acidwith P205 is known. This method, however, has not been suitable fortechnical adoption, some of the reasons being that low yields areobtained and contamination with decomposition products is experienced.

The preparation of unsaturated acid esters by the use of acetoxyderivatives in vapor phase dccomposition has been proposed. Forinstance, a-azetoxy methyl propionate has been decomposed :by vaporphase decomposition to give methyl acrylate. The cost of preparation andpurification of the acetoxy derivative has prevented any widespreadadoption of this method of preparing unsaturated compounds. Processeshave been proposed for preparing unsaturated acids or their derivativesin which a-hydroxymonocarboxylic acids or esters were subjected to theuse of ordinary dehydration catalysts. The application of heat in such aformation of lactides, whizh, on further heating, break down to give analdehyde or ketone and carbon monoxide. In the case of esters, someester interchange also occurs. For instance, ethyl lactate givesconsiderable ethyl lactyl lactate by heating at elevated temperatures.

One object of my invention is to prepare (1,5- unsaturated esters ornitriles directly from the corresponding oc-hYdYOXY compounds. Anotherobject of my invention is to provide a method of accomplishing thisdehydration by a one-step reaction in the vapor phase in the presence orabsence of diluents. A further object of this invention is to provide aprocess for making unsaturated compounds in which only inexpensive andreadily available raw materials are employed. A still further object ofthe invention is to provide a process in which the by-products obtainedare suitable for use in subsequent reactions.

I have found that by subjecting oz-hYdlOXY- monocarboxyli: esters,nitriles, amides or ammonium salts to pyrolysis, together with keteneeither in the presence or absence of diluents deprocess leads to the 2hydration in the vapor phase of the lower molecular weight a-hYdIOXYaliphatic acid derivatives will occur to give the corresponding ae-unsaturated compound. My invention in its broader aspects comprisesthe passage of a mixture of the a-hydroxy'compound and ketene in gaseousform through a pyrolysis tube whereby dehydration occurs. I have foundthat compounds having the following formula may be employed as thestarting material to be dehydrated in accordance with my invention:

R 0 R"-oH2( i-x X being NHz, ONHi, OR or NH( ,CHzR R and R" being alkylor hydrogen sub-stituents.

My invention is particularly adapted for the dehydration of lowmolecular weight compounds of the formula stated such as compoundshaving not more than 8 carbon atoms in a straight chain therein.

I have found that unsaturated nitriles may also be obtained by pyrolysisof saturated a-hYdlOXY aliphatic nitriles such as 'u-hydroxy isobutyronitrile with ketene.

In carrying out the invention other than atmospheric pressures may beused although I have found that the reaction works satisfactorily underordinary pressure conditions. It is desirable that the temperature inthe pyrolysis tube be between 400 and 600 0., the best results havingbeen ob tained in my work at approximately 550 C. The dehydration isconveniently carried out by passing the a-hYdlOXy compound to bedehydrated through a heated tube together with ketene. The a-hYdI'OXYcompound and the ketene may be in pure condition or there may be othermaterialspresent or the u-hYdlOXY compound may be made up of a mixtureof several materials of this nature. It is often desirable to use ketenewhich has decomposition gases present therein as it has been found thatthese gases ordinarily assistin sweeping the pyrolysis chamber. Ifdesired, diluents may be added to the material passing through thepyrolysis tube. For instance, it is particularly desirable in some casesthat acetic anhydride vapors also accompany the vapors of the ketene andthe a-hydroxy compound. In many cases the use of an acetic anhydridediluent has suppressed the formation of undesirable by-products andincreased the conversion of the a-hydroxy compound to the unsaturatedcompound on a single passage through the pyrolysis tube. The pyrolysistube, in general, takes the term of a vessel of tubular shape providedwith means for keeping it at a substantially steady temperature. It isalso desirable that the tube be packed with some inert material givinghigh contact surface such as quartz, pyrex chips, silica gel, carbonrods or the like. The dimensions of the tube and the rate at which thematerials are supplied thereto are dependent upon each other and thelength of tube which is most convenient is a question of choice of theindividual operator. Processes of this type heretofore have employedtubes 40 inches long with an internal diameter of one inch packed with/2 inch mesh particles. With a tube of this size the materials have beenallowed to run in at the rate of 5 grams per minute. With a longer tubeor one in which the gases contact more space in a given amount of timethe rate at which the ases are led into the tube may be increased andvice versa. My process may be carried out continuously by leading themixture of ketene and oz-hYdI'OXY compound into the heated tube at oneend and withdrawing the pyrolysis products at the other. In addition tothe contact surface and free space, the temperature employed in the tubealso has an influence on the rate at which the vapors are led throughthe tube to obtain the best results. If the vapors are led through toofast the temperature is lowered unduly so that the materials to reactare not subjected to the required temperature for reaction or if theyare, are not at that temperature for the desired time. The best resultsin accordance with my process havebeen obtained in a pyrolysis tube inwhich the ratio of the volume of the contact material to the free spacetherein is approximately 3 to 1.

The materials employed as the starting material in my process may becharacterized as esters, nitriles, amides or ammonium salts of monoordialkyl glycollic acids. For instance, some of the compounds which maybe employed with ketene ."in my process are methyl lactate, ethyllactate, a-hydroxymethylisobutyrat lactic amide, ammonium lactate,lactic amide-ammonium lactate mixture, or hydroxyisobutyronitrile, Nacetyl a-hydroxyisobutyramide, methyl-a-hydroxy isobutyrate or, as amatter of fact, any of the lower molecular weight compounds whichcorrespond to the formula given herein. The reaction in accordance withmy invention I have found to be characteristic of the compounds of thistype.

It is desirable in the carrying out of my process in most cases that thematerial which is employed is in concentrated form and free of water. As

pointed out above, this is a dehydration operation and the presence ofwater serves no useful purpose and oftentimes interferes with theobtaining of the best results. In addition, it may cut down the rate ofconversion of the pyrolysis process even in cases where it is notobjectionable. The following examples illustrate my invention:

Example 1 Methyl lactate vapors and ketene in the molar ratio of 1 to1.5 were passed through a cracking tube at a temperature of 550 C. usinga contact time of 6.4 seconds. One mole of methyl lactate gave 28 gramsof methyl acrylate along with some acetaldehyde.

Example 2 One mole of methyl lactate and one mole of acetic anhydridewere preheated to 140-150 C.

and passed into a tube filled with pyrex rings at 550 C. Ketene waspassed in at a molar ratio or 1 to 1 of methyl lactate. The contact timewas '1 seconds. Distillation of the product gave 30 grams of methylacrylate and no low boilers. The residue, upon being repassed, gave 40grams of methyl acrylate. A yield of 81% was obtained.

' Example 3 One mole of methyl lactate and 2 moles of acetic anhydridewere preheated at 150 C. and passed into a tube filled with pyrex chipsat 550 C. Ketene was passed in at a molar ratio of 1.2 to 1 of methyllactate. The distillation gave 64 grams of methyl acrylate. The contacttime was 4 seconds.

Example 4 One mole of lactic amide and one mole of acetic anhydride wereheated to gentle boiling and a homogeneous solution was formed. Thissolution was preheated at -150 C. and passed through the tube at 550 C.using a 6 second contact time and a 2 to 1 molar ratio of ketene toamide. The resulting product was distilled giving 35 grams ofacrylonitrile which is a conversion of 67%. The residue was subjected topyrolysis to get more acrylonitrlle.

Example 5 100 grams of lactic amide-ammonium lactate mixture and 200grams of acetic anhydride were heated to form a solution. This solutionwas preheated to C. and passed through the pyrolysis chamber at 530 C.using a 4 second contact time together with ketene in a molar ratio ofketene to amide of 2 to 1. Distillation of the resulting product gave2'? grams of acrylonitrlle.

Example 6 250 grams of ammonium lactate-lactic amide and 250 grams ofacetic anhydride were heated to form a solution. This was preheated at140 C. and passed into the tube together with ketene which tube had atemperature of 550 C. The ketene was present in a 1 to 1 molar ratio tothe amide. The contact time was 6 seconds. The resulting product wasdistilled and yielded 50 grams of acrylonitrile. I 1

Example 7 25 0 grams of ammonium lactate-lactic amide and 500 grams ofacetic anhydride were heated to form a solution. This was preheated at140-150 C. and mixed with ketene and pyrolyzed at 520-530 C. using a sixsecond contact time. The molar ratio of ketene to amide was 1.5 to 1.The resulting product was distilled and gave 84 grams of acrylonitrlle.

Example 8 5 moles of methyl lactate and 10 molesot anhydrous ammoniawere autoclaved at 100 C. to form the amide. Excess ammonia and methanolwere removed by vacuum distillation. The amide was dissolved in 5 molesof hot acetic anhydride. The mixture was heated at C. and together withketene was passed through a column and into a pyrolysis tube full ofpyrex'chips at a temperature of 550 C. using a 6 second contact Theyield obtained was grams of Example 9 1 mole of lactic amide was passedintoa pyrolysis tube in molten condition and preheated to ISO-160 C.Ketene gas was passed through the pyrolysis chamber in the ratio of 2 to1 to the amide. The pyrolysis was carried out at 550 C. using a 4 secondcontact time. The pyrolysis chamber was packed with pyrex rings. Theproduct was distilled giving 15 grams of acrylonitrile having a boilingpoint of 78 C. at 760 mm. pressure. 50 grams of a-acetoxypropionitrilewere also recovered. 7

Example 10 1 mole of lactic amide in molten condition was passed intothe pyrolysis chamber and preheated to 140-160 C. Ketene was passedthrough the pyrolysis chamber in the ratio of 4 to 1. The contact. timewas seconds and nitrogen was also passed through at the rate of 3 cubicfeet per hour. The pyrolysis was carried out at 520 C. Distillation gave18 grams of acrylonitrile and 70 grams of higher boiling material.

Example 11 170 grams of a-hydroxyisobutyrnitrile (acetone cyanohydrin)was dropped through a preheater at a temperature of 160 C. into apyrolysis chamber packed with pyrex rings. Ketene was then passedthrough the pyrolysis tube in the ratio of 2 to 1. The temperature wasmaintained at 475 C. and the contact time'was 4 seconds and the flow ofnitrogen was 4 cubic feet per hour. The yield was 22% of a-methylacrylonitrile.

I claim:

1. A method of preparing methyl acrylate which comprises subjecting amixture of ketene and methyl lactate to pyrolysis at a temperature of400 to 600 C.

2. A method of preparing methyl acrylate which comprises subjecting amixture of ketene and methyl lactate to pyrolysis at a. temperature of400-600 C. in the absence of a catalyst.

' HUGH J. HAGEMEYER, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Karrer, Organic Chemistry, 1938,p. 198. Van der Burg, Rec. Trav. Chim., vol. 41 (1922), p. 23.

